Synthesis of new indirubin derivatives and their in vitro anticancer activity

The opening of epoxy rings from (2′Z)-N-1-(oxiran-2-ylmethyl)indirubin (2) and (2′Z-3′E)-indirubin-3ʹ-[O-oxiran-2-ylmethyl)oxime] (6) with thiols gave 17 new derivatives of indirubin in good yields. Their structures were elucidated by 1D-, 2D-NMR and HRMS spectra. Screening for anticancer activity was performed with four human cancer cell lines: SW480, LU-1, HepG2 and HL-60 in comparison with indirubin, indirubin-3′-oxime and 6-mercaptopurine. The results showed that cytotoxic and anti-proliferative activities of five derivatives were found in the range of 1.35–19.24 µM. Among synthesized derivatives, 4f showed the strongest activity against all four tested cancer cell lines with IC50 values of 1.65, 2.21, 1.90 and 1.35 µM, respectively.